In general, in a data transmission system, as shown in FIG. 5A, a multiplier multiplies a high-frequency (RF) carrier signal c and a data signal b so as to obtain a modulated signal d to be transmitted.
In this case, as shown in the phase vector graph of FIG. 5B, in addition to the normal vector of the modulated signal d, the obtained modulated signal d contains a carrier leak component of the carrier signal c which leaks into the modulated signal d through the multiplier 1.
A carrier leak phenomenon in which the carrier signal c leaks into the modulated signal d also occurs in a quadrature modulator incorporating two multipliers in the same manner.
When, therefore, the modulated signal d is demodulated into the original data signal b on the receiving side, the original data signal b cannot completely obtained.
FIG. 6 is a block diagram showing the schematic arrangement of a general quadrature modulator conventionally used in a data transmission system including a cellular phone.
As shown in FIG. 6, the externally input carrier signal c formed from, e.g., a sine wave is input to one multiplier 2, and is also input to the other multiplier 4 after the signal is phase-shifted by a 90° phase shifter 3 by 90°.
An in-phase signal I representing an in-phase component of a baseband signal is input to one multiplier 2.
A quadrature signal Q representing a quadrature component of the baseband signal is input to the other multiplier 4.
One multiplier 2 multiplies the carrier signal c and in-phase signal I to output the resultant signal as a product signal d1 to an adder 5.
The other multiplier 4 multiplies the 90° phase-shifted carrier signal c and quadrature signal Q to output the resultant signal as a product signal d2 to the adder 5.
The adder 5 adds the product signals d1 and d2 respectively output from the multipliers 2 and 4, and outputs the resultant signal as a modulated signal a (quadrature-modulated signal) to the exterior.
In this quadrature modulator as well, the product signals d1 and d2 output from the multipliers 2 and 4 respectively contain a leak vector LI leaking from the carrier signal c and a leak vector LQ leaking from the signal obtained by phase-shifting the carrier signal c by 90°, as shown in FIG. 7.
That is, a leak vector VL obtained by adding (vector synthesis) the product signals d1 and d2 is contained in the modulated signal a output from this quadrature modulator.
Therefore, even while the signal levels of the I and Q signals are “0”, the signal level of the modulated signal a is not “0” but is equal to the level of the absolute value of the leak vector VL.
In order to prevent the leak vector VL from being contained in the modulated signal a, an adjustment vector VC in the opposite direction to the leak vector VL may be applied (added) thereto, as shown in FIG. 7.
In order to realize the adjustment vector VC, a DC adjustment voltage EIC and DC adjustment voltage EQC may be respectively added to the I and Q signals in advance in the quadrature modulator shown in FIG. 6.
More specifically, as shown in FIG. 8, adders 6 and 7 are inserted in the signal paths for the I and Q signal input to the quadrature modulator.
The same reference numeral as in FIG. 6 denote the same parts in FIG. 8.
Variable voltage sources 8 and 9 respectively apply the above DC adjustment voltages EIC and EQC to the adders 6 and 7.
A sequence for setting operation for the adjustment voltages EIC and EQC which is executed by an operator will be described next in detail.
First of all, the signal levels of the I and Q signals are set to “0” by the operator.
In this state, the operator measures the signal level of the modulated signal a output from the quadrature modulator with a measurement instrument capable of measuring a very low level with high precision, such as a spectrum analyzer 11, through a high-frequency circuit 10 including an amplifier.
The operator then adjusts voltages to be applied to the I and Q signals by operating the variable voltage sources 8 and 9 with an operating section 12 while observing the signal level of the modulated signal a displayed on the spectrum analyzer 11.
More specifically, the operator searches for a combination of voltages to be applied which sets the signal level of the modulated signal a to “0” or minimizes it, and sets the respective voltages of the combination as the adjustment voltages EIC and EQC.
However, the technique of canceling out the carrier leak contained in the modulated signal a output from the quadrature modulator by using the adjustment voltages EIC and EQC added to the I and Q signals as shown in FIG. 8 still has the following problems to be solved.
While the signal levels of the I and Q signals are set at “0”, the modulated signal a output from the quadrature modulator contains only a carrier leak component, and hence its signal level is very low.
In order to obtain the adjustment voltages EIC and EQC with high precision which are used to cancel out the carrier leak contained in the modulated signal a, the very low level of the modulated signal a must be measured with high precision.
A high-precision measurement instrument such as the spectrum analyzer 11 described above must be prepared to measure the very low signal level of the modulated signal a with high precision.
This greatly increases the equipment cost. It is therefore almost impossible in terms of cost to incorporate such a carrier leak adjustment function in, for example, a signal generator incorporating a quadrature modulator.
In addition, an operator makes a search, by trial and error, for a combination of voltages to be added to the I and Q signal so as to set the signal level of the modulated signal a to “0” or minimize it.
This operation requires many repetitive adjustments, and hence it is difficult to automate such a carrier leak adjustment function and incorporate it in, for example, a signal generator incorporating a quadrature modulator.
Furthermore, a search for a combination of voltages to be added to the I and Q signals so as to set the signal level of the modulated signal a to “0” or minimize it has been executed by experience and intuition of a skilled operator. Therefore, this adjustment operation requires an unskilled operator to perform extremely inefficient operation demanding much time and effort.
According to a method of calibrating a vector modulator disclosed in U.S. Pat. No. 4,717,894 as a prior art, a technique of automating carrier leak calibration is disclosed, which is similar to the carrier leak adjustment function in the quadrature modulator described above.
According to this prior art, the carrier leak calibration function works as follows. First of all, DC voltages to be applied to the I and Q signals, e.g., two DC voltages to be applied to the I phase when the I phase voltage is changed to set a given RF output level are calculated. The median between these two DC voltages is then obtained.
With regard to the Q phase, two DC voltages to be applied to the Q phase when a given RF output level is set are obtained by the same procedures as those for the I phase, and the median between the voltages is obtained.
In this prior art, a carrier leak in the vector modulator is automatically calibrated by repeating these procedures.
This prior art is, however, based on not only carrier leak calibration for the vector modulator but also orthogonality adjustment of an RF carrier signal generated by an LO (Local Oscillator). For this reason, combinations of four fixed voltages I−, I+, Q−, and Q+ are respectively applied to the I and Q signals, and the values of the resultant RF output levels themselves are read to perform calculations.
In this prior art therefore, as a level detector for reading the value of an RF output level itself, a digital/analog (D/A) converter itself or the like capable of measuring a level itself is required to satisfy such procedures. Accordingly, this further complicates the overall arrangement, posing a problem in terms of cost.
Under the circumstance, a problem arises in applying such a carrier leak calibration function based on the prior art to the carrier leak adjustment function of the above quadrature modulator.